For example, in cold rolling of steel sheet, from the viewpoint of the stabilization of the rolling operation, shape and surface quality of the product, prevention of seizure, roll lifetime, etc., it is necessary to maintain the friction coefficient between the rolled material (steel sheet) and work rolls at a suitable value. To obtain a suitable friction coefficient, a lubrication oil suitable for the grade and dimension of the rolled sheet and the rolling conditions is selected and is fed at the inlet side of the rolling stand to the rolled steel sheet and the rolls. In tandem cold rolling mill, in general, emulsion lubrication is used.
To obtain a suitable friction coefficient, increasing the emulsion supplying rate or emulsion concentration is an effective means for improving the lubricity and reducing the friction coefficient, but this invites an increase in cost. Further, there is a limit to the increase in the emulsion supplying rate or emulsion concentration in the current state due to limitations in facilities. As the limitation in facilities, for example, in the case of high concentration emulsion, sometimes the pipes become clogged or the capacity of an agitator in the tank impairs the homogeneity of the high concentration emulsion. Further, the upper limit of the emulsion supplying rate is determined by the pump capacity.
Recently, high tensile steel, TRIP steel, and other materials called hard to produce materials are being rolled in an increasing trend. With hard to produce materials, the rolling load becomes higher, so there is a need to reduce the friction coefficient and reduce the rolling load at the front-end stands (e.g. 1st and/or 2nd stand) in the tandem cold rolling mill and to prevent seizure by reducing the friction coefficient and suppressing the friction heat in the rear-end stands which rolling rate becomes higher. That is, there is a need to reduce the friction coefficient in the entire rolling rate region compared with a mild carbon steel when rolling a hard to produce steel sheet.
If schematically showing the case of use of a lubrication oil A where the friction coefficient becomes within the allowable range with respect to a mild steel, the result becomes as shown in FIG. 9. The lower limit of the allowable range of the friction coefficient is the limit at which the friction coefficient cannot be lowered further due to the performance of the lubrication oil, restrictive conditions of the facility, etc. explained above. Further, slip occurs even if there are no problems in restrictions of the facilities, so sometimes the friction coefficient cannot be lowered further. On the other hand, the upper limit is determined by the seizure resistance in the boundary friction region of the lubrication oil. From experience with operations up to now, the upper limit has been determined. The rolling conditions are set so that the friction coefficient becomes somewhat smaller than that. Up until now, mostly mild steel had been rolled, so lubrication oil A alone was able to handle it. However, as clear from FIG. 9, to roll super high tensile steel with a tensile strength of for example 1270 MPa or more, lubrication oil A alone was not sufficient to obtain a suitable friction coefficient.
To solve this problem, the method of using a plurality of types of lubrication oil may be considered. For example, there is the method of preparing a low concentration and a high concentration lubrication oil by the same lubrication oil and supplying it to different supplying locations (for example, see Japanese Patent Publication (A) No. 59-33023) or the method of selective use in accordance with the steel sheet thickness (for example, see Japanese Patent Publication (A) No. 8-155510). However, as with these methods, even if using the same lubrication oil and changing the concentration, if considering the limitations in facilities of the rolling mill or cost, it is difficult to handle the current plurality of rolled steel sheets.
Further, in another method of supplying lubrication oil, the method of preparing four tanks, prescribing three different types of lubrication oil, and selectively using them in accordance with the steel sheet thickness has been proposed (for example, see Japanese Patent Publication (A) No. 59-199109). This method uses four tanks and three types of lubrication oil and a detergent solution, but there is no description of the film thickness or the friction coefficient. Further, the method of classification of the grade and lubrication oil is also rough. There are the problems that it is difficult to control lubrication sufficiently to meet all the strict requirements for surface quality in recent years or all the small lots of various types of rolled steel sheets and fine control is difficult.
Further, there is also the method of changing the mixing ratio of at least two types of lubrication oil to change the composition of the lubrication oil in accordance with the quality characteristics required from the hot rolled steel strip for each hot rolled steel strip and supplying the rolling oil comprised of the lubrication oil and water to at least one rolling stand (for example, see Japanese Patent Publication (A) No. 2000-351002). With this method, the lubrication oil supplying rate is controlled in accordance with only the quality characteristics required, so fine control was not possible.